CNTs
Carbon nanotubes (CNTs), their remarkable properties and potential properties, and methods of making them have been known for many years. However their industrial uses are still very limited, largely because of processing and handling issues. They can be made by various processes but the main ones are arc discharge from carbon-containing electrodes, and vapour-phase deposition of carbon, by laser ablation or CVD, onto metal catalyst particles. These methods can make CNTs of single-wall and multi-wall types (SWCNTs and MWCNTs) and are well-known to the skilled person.
The resulting CNTs are usually contaminated with residues of one or more of catalyst, amorphous carbon and (usually undesired) closed fullerenes and these residues tend to bind the CNTs together. Moreover CNTs, like most nanoparticles, have a strong tendency to agglomerate under the influence of van der Waals' forces, because of their extremely high specific surface area. With CNTs this is exacerbated by their very high aspect ratio leading to extensive tangling and winding, and structures such as lumps, granules, tangled bundles or “ropes” of twisted CNTs. Most high volume production batches of CNTs made by the above mentioned methods consist essentially of such tangled and contaminated aggregate structures.
Many of the important uses envisaged to exploit the special properties of CNTs involve dispersing them in matrix or binder materials. As an intermediate handling stage, dispersal in a liquid vehicle such as water or organic solvent is envisaged (especially in view of the health risk presented by the dry particles). However the routine existence of the bulk product in the form of tangled agglomerates and their lack of relative chemical affinity for the vehicle or matrix presents a formidable obstacle to dispersion. Where dispersions can be formed these tend to be dispersions of agglomerates, so that the properties of the CNTs themselves are scarcely made available.
It is known to functionalise and disperse nanoparticle aggregates in general, and CNTs in particular, using an aggressive combination of mechanical and chemical treatments, e.g. by boiling in acid to decompose contaminants and functionalise the carbon surface, and breaking apart the aggregates using high-shear methods such as milling, grinding or ultrasonication. The dispersion can then be stabilised to some extent in a liquid vehicle by means of surfactants or other colloid chemistry methods. This has met with some success, but these techniques for functionalising the particles remain highly inefficient, inconvenient and expensive as far as industrial application is concerned. Moreover they still achieve only modest levels of de-aggregation of the individual CNTs. Typically the CNTs are still twisted into ropes, and often have quite severe structural damage to the carbon layers (graphene layers of the CNT wall) as well as shortening of the tubes, with consequent loss of valuable CNT properties. Each chemically modified (functionalised) site represents a structural defect, with a missing carbon atom or bond re-arrangement.
CNTs also present a real or perceived health hazard if inhaled or in general if they contact permeable body membranes. Accordingly, despite widespread knowledge of their potential properties and of ways of making them, they have found limited industrial application.
Graphene
Separately, graphene is known as the single-layer hexagonal form of carbon, corresponding to a single layer of the graphite structure but with properties exceeding graphite's because of the absence of neighbouring layers. Graphene layers can be made to quite large sizes by careful mechanical “exfoliation” or intercalation utilising an oxidant such as concentrated sulphuric acid and nitric acid, from graphite, by reduction of exfoliated graphene oxide, or by epitaxial growth on substrates of other materials. However the known methods are laborious and expensive.
Use of graphite-based materials, with their characteristic layer structure (a graphene sheet being a hexagonal lattice of carbon atoms, and graphite being a stacked series of these sheets) becomes relatively attractive in view of the drawbacks of CNTs. Even when extremely thin (one or a few layers) they are more particle-like than CNTs and, as a consequence, safer and less difficult to handle and disperse. Even more than with CNTs, however, there has been no easy commercial-scale supply of readily-useable graphene material. While CNTs have been known for many years, the first successful preparations of true graphene have occurred only recently. Synthetic laboratory-grown graphene is available only in tiny quantities at enormous cost. A number of important practical applications exist but their implementation is necessarily very limited.
The other methods available to produce graphene material are as follows. Mined graphite is used as starting material. A step of intercalating to enable exfoliation may bee chemical intercalating, electrochemical intercalating, gas phase intercalating, liquid phase intercalating, supercritical fluid intercalating, or a combination thereof. Chemical intercalating may expose the graphite to sulphuric acid, sulphonic acid, nitric acid, a carboxylic acid, a metal chloride solution, a metal-halogen compound, halogen liquid or vapor, potassium permanganate, alkali nitrate, alkali perchlorate, an oxidizing agent, or a combination thereof. Halogens may also be used to intercalate, e.g. from bromine, iodine, iodine chloride, iodine bromide, bromine chloride, iodine pentafluoride, bromine trifluoride, chlorine trifluoride, phosphorus trichloride, tetrachloride, tribromide, triiodide, or combination thereof.
Electrochemical intercalating may use nitric acid or a carboxylic acid as both electrolyte and intercalate source, with a current density in the range of 50 to 600 A/m2 at the graphite, which is used as an electrode.
The step of exfoliating the intercalated graphite may comprise exposing the intercalated structure to a temperature in the range of 150° C. to 1,100° C. When the intercalating uses an acid as intercalate, the exfoliating typically comprises exposing the intercalated graphite to a temperature in the range of 600° C. to 1,100° C. When intercalating uses a halogen or halogen compound the exfoliating typically comprises exposing the intercalated graphite to a temperature in the range of 50° C. to 350° C.